Control of externally induced current in implantable medical devices

ABSTRACT

A current limiting apparatus that is adapted to be operatively connected as part of a conductive loop formed by a medical device implanted within a living organism having electrically excitable tissue. This apparatus limits unwanted current in the conductive loop that may be induced by a significant level of an external signal such as a time-alternating electromagnetic field. This apparatus includes a switch for introducing a high impedance into the conductive loop when the switch is turned off. The apparatus also includes a control circuit that controls the switch to be turned on when the conductive loop should be closed to stimulate the electrically excitable tissue for a therapeutic effect. The control circuit turns the switch off whenever the living organism enters an area having a significant level of external signal that may induce unwanted current in the conductive loop to limit the unwanted current. The living organism can manually initiate turning off the switch when the living organism is about to enter an area having such an external signal. Alternatively, such an external signal is sensed to turn the switch off automatically when such an external signal is sensed.

This is a divisional of application Ser. No. 08/847,642, filed Apr. 30,1997, for which priority is claimed.

BACKGROUND OF THF INVENTION

1. Field of the Invention

This invention relates generally to implantable medical devices, andmore particularly to a method and apparatus for limiting unwantedcurrent flow through electrically excitable tissue resulting fromapplication of an external signal on an implanted medical device.

2. Description of the Related Art

The use of implantable medical devices for electrical stimulation ofelectrically excitable tissue is well known in the medical arts.Electrical stimulation of brain tissue has been used for tremorsuppression. Moreover, electrical stimulation of peripheral nerve tissuehas been used to promote blood circulation in patients having peripheralvascular disease. Additionally, electrical stimulation of the brain andnerve tissue of the spinal cord has been used for pain management. Insuch devices, electrodes deliver the stimulation signal to theelectrically excitable tissue. The electrodes are operatively connectedto an implantable pulse generator which is packaged in a case that isadapted to be implantable. Those electrodes are coupled to that pulsegenerator by a conductive lead wire.

A user having such an implanted medical device during normal lifeactivities may be forced to go through a time-alternatingelectromagnetic field. Prevalent examples of sources of electromagneticfield are Electronic Article Surveillance (EAS) systems. Such systemsdetect theft and are found in the exit doorways of many stores andlibraries. EAS systems typically emit an AC electromagnetic field fordetecting theft of articles that have an attached electromagnetic tag.

When the user having an implanted medical device walks through an EASsystem that emits a significant level of AC electromagnetic field,current may be induced in the electrically excitable tissue by theFaraday effect. According to the Faraday effect, when a conductive loopis disposed within a time-alternating electromagnetic field, a currentis induced in that conductive loop. With the implanted medical devicewithin the user, the conductive loop consists of the stimulationelectrodes, the lead wire, the implantable pulse generator having acase, and the conductive medium of the body including the electricallyexcitable tissue between the stimulation electrodes and the case. Thecase in some implantable medical devices acts as a reference returnelectrode with respect to the stimulation electrodes and is composed ofelectrically conductive material.

The user may experience unwanted physiological effects from the currentthat is induced in the electrically excitable tissue of that conductiveloop. This current may cause uncontrolled excitation of electricallyexcitable tissue which can lead to pain sensations and unwanted motorresponses for the user. Thus, means for controlling that induced currentis desired.

OBJECTS OF THE INVENTION

Accordingly, a primary object of the present invention is to limit thecurrent that may be induced within the conductive loop formed by animplanted medical device when a significant level of external signalsuch as an electromagnetic field is present. The induced current islimited automatically upon sensing a significant level of an externalsignal or manually with the user controlling the opening of theconductive loop when the user is about to enter an area having asignificant level of external signal.

SUMMARY OF THE INVENTION

In a principal aspect, the present invention takes the form of anapparatus and method for limiting current flow, induced when the levelof an external signal is greater than an external signal thresholdlevel, in a conductive loop formed by a medical device implanted withina living organism having electrically excitable tissue. The presentinvention includes a switch that is adapted to be operatively connectedwithin the conductive loop which includes the implanted medical deviceand the electrically excitable tissue. Additionally, a control circuitcontrols the switch to turn the switch on when the medical device isstimulating the electrically excitable tissue to achieve a therapeuticeffect if the level of the external signal is less than the externalsignal threshold level. Moreover, the control circuit controls theswitch to turn off whenever the level of the external signal is greaterthan the external signal threshold level, to thereby introduce a highimpedance into the conductive loop.

The present invention can be applied to particular advantage when usedwith a sensor for sensing when the level of the external signal isgreater than the external signal threshold level. The control circuitwould automatically turn the switch off when the sensor senses that thelevel of the external signal is greater than the external signalthreshold level. Alternatively, the living organism, having theimplanted medical device, manually initiates the opening of theconductive loop. In this manner, the conductive loop can be openedmanually to limit unwanted current that can result from application of asignificant level of external signal on a closed conductive loop.

These and other features and advantages of the present invention will bebetter understood by considering the following detailed description ofthe invention which is presented with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an electrical stimulation system which forms a closedconductive loop when implanted within a human body;

FIG. 1B shows the electrical stimulation system of FIG. 1A with thefurther inclusion of a switch for opening the conductive loop formed bythe implanted electrical stimulation system, according to a preferredembodiment of the present invention;

FIG. 2 shows simple switch devices and the cross sections of thoseswitch devices fabricated with semiconductor integrated circuittechnology;

FIG. 3 shows a current limiting circuit according to a preferredembodiment of the present invention; and

FIG. 4 shows an external signal sensor circuit according to a preferredembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1A, an electrical stimulation system 100 comprisesstimulation electrodes 102 implanted near electrically excitable tissue104 of a human body 106 of a user. The electrically excitable tissue inFIG. 1A is nerve tissue within the spinal column. However, electrodes ofimplanted medical devices are also commonly implanted near other typesof nerve tissue such as peripheral nerve tissue and brain tissue. Theelectrical stimulation system 100 also includes an implantable pulsegenerator 108 which provides the stimulation signals to be applied onthe stimulation electrodes 102 via a conductive lead wire 110.

A programmer 112 sends messages to the implantable pulse generator 108to modify the stimulation signals to be applied on the electrodes 102.The programmer controls the pulse generator by radio frequencycommunication to the pulse generator via an antenna 114 andcorresponding receiver in the generator 108. An example of such anelectrical stimulation system is the ITREL II system available fromMedtronic, Inc., Minneapolis, Minn. While the preferred system employsfully implanted elements, systems employing partially implantedgenerators and radio-frequency coupling may also be used in the practiceof the present invention. Such systems are also available fromMedtronic, Inc. under the trademarks X-trel and Mattrix.

The electrical stimulation system 100 of FIG. 1A is implanted to cause atherapeutic effect of pain reduction by inducing an action potential inthe electrically excitable tissue 104. The implantable pulse generatorgenerates the stimulation signals to be applied on the electrodes 102.For example, the induced action potential in the nerve fibers of thespinal cord can reduce pain experienced by the human body 106.

The preferred embodiment of the present invention is described for theelectrical stimulation system 100 used to achieve the therapeutic effectof pain reduction in the human body 106. However, it should beappreciated that the present invention can be used with otherimplantable medical devices for achieving other therapeutic effectsaside from just pain management. Additionally, it should be appreciatedthat the present invention can be used for medical devices implanted inother living organisms aside from just a human body.

In the implanted medical device of FIG. 1A, a conductive loop is formedby the stimulation electrodes 102, the lead wire 110, the implantablepulse generator 108 having a conductive case, and a conductive medium ofthe human body 106 between the stimulation electrodes and the conductivecase of the implantable pulse generator. The case of the implantablepulse generator can act as a reference electrode with respect to thestimulation electrodes. The conductive medium of the human body includesthe electrically excitable tissue 104.

When the user having such an implanted medical device enters an areahaving a significant level of time-alternating electromagnetic field, acurrent may be induced in the conductive loop formed by the implantedmedical device and by the electrically excitable tissue in accordancewith the Faraday effect. According to the Faraday effect, when atime-alternating electromagnetic field is applied on a conductive loop,a current is induced in that conductive loop. Common sources oftime-alternating electromagnetic field are Electronic ArticleSurveillance (EAS) systems installed on exit doorways of many stores andlibraries. Other sources of time-alternating electromagnetic fieldsinclude Nuclear Magnetic Resonance Imaging (NMRI) systems, securityscreening corridors found in airports, and cellular telephones.

When such an external signal induces a current in the conductive loopand in turn the electrically excitable tissue, the user may experiencepain and uncontrollable motor responses. Thus, this induced current isunwanted, and the present invention limits this unwanted current.

In the implanted medical device system 100 of FIG. 1A, the implantablepulse generator 108 generates stimulation signals to be applied on thestimulation electrodes 102. The conductive loop formed by the implantedmedical device and by the electrically excitable tissue is closed whenstimulation signals are applied on the stimulation electrodes to inducean action potential in the electrically excitable tissue 104. Thataction potential creates a current in the electrically excitable tissueto cause a therapeutic effect. In that case, the conductive loop isclosed such that current can flow in the conductive loop.

When the user enters an area with a significant level of external signalsuch as a time-alternating electromagnetic field, that external signalmay induce unwanted current in that conductive loop. In that case, theconductive loop is opened to limit the induced unwanted current. Thegeneral embodiment of the present invention opens that conductive loopby introducing a switch into that conductive loop.

Referring to FIG. 1B, the electrical stimulation system 100 of FIG. 1Ais shown with similar elements having the same number label. Theelectrical stimulation system 100 of FIG. 1B further includes a switch116 operatively connected between the case of the pulse generator 108and the stimulation electrodes 102. However, the switch 116 can beinserted within any part of the conductive loop formed by the implantedmedical device for proper operation of the present. invention. In thepreferred embodiment of the present invention, the switch 116 and asensor circuit 118 used with that switch are disposed inside theimplantable pulse generator 108.

The switch 116 is opened to prevent flow of unwanted current induced inthe conductive loop by an external signal. The switch can be openedautomatically upon sensing a significant level of external signal. Thesensor circuit 118 is operatively connected to the switch 116 to openthat switch when the sensor circuit senses a significant level of theexternal signal.

The sensor circuit 118 can sense the level of the external signaldirectly, and in that case, the sensor circuit can be located anywhereon or near the human body 106. In an alternative embodiment, the sensorcan detect the level of unwanted current within the conductive loopformed by the medical device. A significant level of this unwantedcurrent is an indirect indication of the presence of a significant levelof external signal. Alternatively, the switch 116 can be aelectromagnetically sensitive switch that opens when the level of theexternal signal is greater than an external signal threshold level, andin that case, the sensor circuit 118 is not required.

The switch closes when the level of the external signal is less than theexternal signal threshold level. External signals such as radiofrequency signals are prevalent in the surroundings of a user. However,when the level of external signal exceeds the external signal thresholdlevel, that external signal may induce enough unwanted current in theclosed loop to cause undesirable physiological effects.

In an alternative embodiment, the user having the implanted medicaldevice can actively open the switch 116. For example, if the switch weremagnetically sensitive, then when the user is about to enter an areawhere the level of the external signal is greater than the externalsignal threshold level, the user brings a magnet near the switch 116 inorder to manually open the switch 116. When the user perceives that asignificant level of external signal is no longer present, the useragain brings the magnet near the switch 116 to close the switch 116. Inthat embodiment also, the sensor circuit 118 is not required.

Referring to FIG. 2, two common means for implementing the switch 116 inCMOS IC technology are the NMOSFET 200 and the PMOSFET 250. The NMOSFET(N-Channel Metal Oxide Semiconductor Field Effect Transistor) has across section 200A, and the PMOSFET (P-Channel Metal Oxide SemiconductorField Effect Transistor) 250 has a cross section 250A.

The cross section 200A of the NMOSFET shows a N-type doped drain region202A that forms a first drain terminal 202, a conductive gate region204A that forms a first gate terminal 204, and a N-type doped sourceregion 206A that forms a first source terminal 206. The conductive gateregion 204A sits on top of a silicon dioxide layer 210. Also, a P-wellregion 208A that forms a P-well terminal 208 is also shown. The P-wellis a P-type doped region and sits within a N-type substrate region 212.

Similarly, the cross section 250A of the PMOSFET shows a P-type dopedsource region 252A that forms a second source terminal 252, a conductivegate region 254A that forms a second gate terminal 254, and a P-typedoped drain region 256A that forms a second drain terminal 256. Theconductive gate region 254A sits on top of a silicon dioxide layer 260.Those P-type doped drain and source regions sit within a N-type dopedsubstrate region 258A that forms a N-substrate terminal 258. With theuse of such devices, the switch 116 and the sensor circuit 118 forcontrolling the switch are located within the implantable pulsegenerator 108.

Either the NMOSFET or the PMOSFET can be inserted as the switch 116 ofFIG. 1B. The gate terminal of such a switch device is controlled toclose the conductive loop when the electrically excitable tissue isstimulated for therapeutic effect. Also the gate terminal of such aswitch device is controlled to open the conductive loop when the user isabout to enter an area having a significant level of external signal.

However, if only a switch device such as one of the NMOSFET or thePMOSFET were inserted in the conductive loop, current may still beinduced in the conductive loop even when the switch device is turnedoff. Parasitic current can still flow in such a switch device even whenthe switch device is turned off Referring to FIGS. 1B and 2, when asignificant level of an external signal such as a time alternatingelectromagnetic field is applied on the medical device 100 implantedwithin the body 106, current may be induced in the conductive loopformed by the implanted medical device.

When the switch is opened to limit current flow, a charge build-up cancause a voltage change at points in the conductive loop. For example,the case holding the implantable pulse generator 108 has been shown tohave a voltage build-up as high as ±8 Volts when a significant level ofexternal signal is present. Assume that the drain terminal of a NMOSFETwere operatively connected to that case and that −8V were to build up onthat case. Then, referring to the cross section 200A of the NMOSFET, anunwanted parasitic current may flow from a forward-biased P-N junctionformed by the P-well region 208A and the N-type doped drain region 202Athat is operatively connected to the case. Similarly, assume that thedrain terminal of a PMOSFET were operatively connected to that case andthat +8V were to build up on that case. Then, referring to the crosssection 250A of the PMOSFET, an unwanted parasitic current may flow froma forward-biased P-N junction formed by the N-type substrate region 258Aand the P-type doped drain 252A that is operatively connected to thecase.

Thus, simply a NMOSFET or a PMOSFET may not sufficiently limit theunwanted current that may be induced by an external signal. FIG. 3 showsa current limiting circuit 300 that can limit unwanted current even whenthere is a voltage build-up at a node within the conductive loop. Thecurrent limiting circuit 300 of FIG. 3 comprises a switch 302 shownwithin dashed lines and a control circuit 304 shown within dashed lines.

The switch 302 includes a PMOSFET 306 as a first switch device and aNMOSFET 308 as a second switch device. The PMOSFET has a first drainterminal 310, a first gate terminal 312, a first source terminal 314,and a N-type substrate terminal 316. The NMOSFET has a second drainterminal 318, a second gate terminal 320, a second source terminal 322,and a P-well terminal 324. The PMOSFET and the NMOSFET have the samecross section as those shown in FIG. 2. The first source terminal 314 ofthe PMOSFET is operatively connected to a positive power supply 326 viaa first coupling capacitor 328. The N-substrate terminal 316 of thePMOSFET is also operatively connected to the positive power supply. Thefirst drain terminal 310 of the PMOSFET is operatively connected to thesecond source terminal 322 of the NMOSFET at a node 330.

The second drain terminal 318 of the NMOSFET is operatively connected tothe output node 332 of the current limiting circuit 300. This outputnode is operatively connected to the implanted medical device. Forexample, in the implanted electrical stimulation system of FIG. 1B, thecurrent limiting circuit 300 is part of the implantable pulse generator,and the output node 332 is operatively connected to the case of theimplantable pulse generator 108. The P-well terminal 324 of the NMOSFET308 is operatively connected to the second source terminal 322.

The control circuit 304 is operatively connected to the first gateterminal 312 of the PMOSFET 306 and to the second gate terminal 320 ofthe NMOSFET 308 to bias those switch devices to be on or off. Thecontrol circuit includes a first NOR gate 334 having a first controlterminal 336 as an input and a second control terminal 338 as an input.The control circuit also includes a second NOR gate 340 having thesecond control terminal 338 as an input and a third control terminal 342as an input. The output of the first NOR gate 334 and the output of thesecond NOR gate 340 are inputs to a third NOR gate 344.

The output of the third NOR gate 344 is operatively connected to thefirst gate terminal 312 of the PMOSFET 306 through a first inverter 346and a second inverter 348. The output of the third NOR gate 344 is alsooperatively connected to the second gate terminal 320 of the NMOSFET 308through a third inverter 350 and through a pair of a second couplingcapacitor 352 and a third coupling capacitor 354 which are connected inparallel.

The control circuit also includes a biasing NMOSFET 356 having a thirddrain terminal 358, a third gate terminal 360, a third source terminal362, and a third P-well terminal 364. The drain terminal of thattransistor is diode connected to its gate terminal, and both its drainand gate terminals are connected to the positive power supply 326. Inaddition, the P-well terminal 364 of that transistor is operativelyconnected to the positive power supply to form another forward biaseddiode connected between the P-well and the N-type doped source ofNMOSFET 356. That source terminal is operatively connected to the secondgate terminal 320 of NMOSFET 308.

The operation of the current limiting circuit 300 of FIG. 3 is nowdescribed. Referring to the implanted medical device 100 of FIG. 1B, theoutput node 332 of the current limiting circuit 300 is operativelyconnected to the case of the implantable pulse generator 108. Thepositive power supply 326 is also part of the implantable pulsegenerator which is operatively coupled to the stimulation electrodes102. Thus, the switch 302 is connected as part of the conductive loopformed by the implanted electrical stimulation system 100. The switch isthen connected in series between the stimulation electrodes and the caseof the implantable pulse generator 108 in that conductive loop.

The current limiting circuit operates to close the conductive loop byturning on both the PMOSFET 306 and the NMOSFET 308 of the switch 302when the implanted medical device stimulates the electrically excitabletissue to achieve a therapeutic effect. The conductive loop is opened byintroducing a high impedance at the output node 332 when the level ofexternal signal such as a time-alternating electromagnetic field isgreater than an external signal threshold level. This high impedance isgenerated by maintaining both of the PMOSFET 306 and the NMOSFET 308 offwhenever the level of an external signal is greater than the externalsignal threshold level.

The first control terminal 336 receives a first control signal, and thethird control terminal 342 receives a second control signal. The secondcontrol terminal 338 receives an over-sense control signal. The firstcontrol signal goes low when stimulation signals are applied on theelectrically excitable tissue to source current into the tissue. Thesecond control signal goes low when stimulation signals are applied onthe electrically excitable tissue to sink current out of the tissue.This electrical stimulation of the tissue by the implanted medicaldevice causes beneficial therapeutic effects. Accordingly, theconductive loop formed by the implanted medical device is closed duringthis stimulation process.

Referring to FIG. 3, for now assume that the over-sense control signalapplied on the second control terminal 338 is tied low. Then, wheneither one of the first control signal applied on the first controlterminal 336 or the second control signal applied on the second controlterminal 342 goes low, the output of the third NOR gate 344 goes low. Inturn, the first gate terminal 312 of the PMOSFET 306 goes low, and thesecond gate terminal 320 of the NMOSFET 308 goes high. The diode formedby the diode connection of the biasing NMOSFET 356 turns off. A lowvoltage on the gate terminal of the PMOSFET 306 turns on that PMOSFET.

A high voltage on the gate terminal of the NMOSFET 308 turns on thatNMOSFET. When the output of the third NOR gate 344 was high, the diodeformed by the biasing NMOSFET 356 was turned on, and the voltage at thegate terminal of the NMOSFET 308 was at the positive power supplyvoltage V_(DD) minus the diode drop of the NMOSFET 356 V_(th),(V_(DD)−V_(th)). Then when the output of the third NOR gate 344 goeslow, the voltage at the output of the inverter 350 goes high, the diodeformed by NMOSFET 356 turns off, and the voltage on the gate terminal ofNMOSFET 308 rises from V_(DD)−V_(th) toV_(DD)−V_(th)+V_(DD)=2V_(DD)−V_(th). This high voltage on the gateterminal of NMOSFET 308 ensures that this NMOSFET remains on in the caseeither one of the first control signal applied on the first controlterminal 336 or the second control signal applied on the second controlterminal 342 goes low.

In this manner, both the PMOSFET and the NMOSFET within the switch 302are on. In that case, the conductive loop is closed such that currentflows within that loop to achieve a therapeutic effect.

The conductive loop formed by the implanted medical device is openedwith a high impedance introduced into the conductive loop when theelectrically excitable tissue is not being stimulated for therapeuticeffect. In that condition, both the first control signal and the secondcontrol signal are set high. With those control signals, the output ofthe third NOR gate 344 goes high.

In turn, the first gate terminal 312 of the PMOSFET 306 goes high. Thediode formed by the diode connection of the biasing NMOSFET 356 turnson. Thus, the second gate terminal 320 of the NMOSFET 308 is at a diodevoltage drop, Vth, from the voltage of the positive power Supply,V_(DD). A high voltage on the gate terminal of the PMOSFET 306 turnsthat PMOSFET off. A voltage of a gate to source voltage drop V_(th) ofthe diode-connected transistor 356 from the voltage on the positivepower supply, V_(DD), applied on the gate terminal of the NMOSFET 308keeps that NMOSFET off.

In addition, the current limiting circuit 300 of FIG. 3 maintains a highimpedance in the conductive loop formed by the implanted medical deviceeven when there is a voltage build-up at one of the nodes in theconductive loop. In the current limiting circuit 300, the output node332 of the switch 302 is lied to the conductive case of the implantablepulse generator 108 of FIG. 1B. When an external signal such as atime-alternating magnetic field is present, induced current in theconductive loop can cause a voltage build-up on the conductive case ofthe implantable pulse generator. This voltage build-up can result involtages as high as ±8V at the case. In contrast to the simple switchdevices of FIG. 2, the current limiting circuit 300 can maintain thehigh impedance within the conductive loop even with such a voltagebuild-up.

For example, with the output node 332 tied to the case of the pulsegenerator 108, assume that the voltage on that case and in turn on theoutput node has built up to −8V. Referring to FIGS. 2 and 3, with such ahigh negative voltage on the output node, the NMOSFET 308 may have aforward biased PN junction between the P-well region and the N-typedoped drain of that NMOSFET. Thus, unwanted current flows in thatparasitic path. In addition, since MOSFET devices in CMOS technology aresymmetrical, the N-type doped region of NMOSFET 308 tied to the outputnode can act as a source. With −8V applied on that terminal, NMOSFET 308may turn on.

However, note that the PMOSFET 306 of the switching circuit is kept offwith its source to gate voltage V_(SG) still being sufficiently low.This PMOSFET is in the path of the parasitic PN junction of the NMOSFET308 and in the path of the drain to source terminals of that NMOSFET.Thus, the PMOSFET blocks the unwanted parasitic current caused by thehigh negative voltage build-up on the case of the implantable pulsegenerator 108.

Similarly, assume that the voltage on the output node 332 has built upto a high positive voltage such as +8V. Referring to FIGS. 2 and 3, withsuch a high positive voltage, the PMOSFET 306 has a parasitic forwardbiased PN junction between the P-type doped drain region and theN-substrate region. Thus, unwanted current flows in that parasitic path.

However, note that NMOSFET 308 is in the path of that parasitic PNjunction. The NMOSFET blocks the parasitic current in that PN junction.Note that the N-substrate 316 of the PMOSFET is tied to the positivepower supply. Thus, for the parasitic PN junction to conduct thatunwanted current, the drain terminal 310 of the PMOSFET must be set at avoltage higher than the voltage of the positive power supply V_(DD) by athreshold voltage of that PN junction, V_(th).

That drain terminal 310 of the PMOSFET 306 is operatively connected tothe source terminal 322 of the NMOSFET 308. The gate terminal 320 of theNMOSFET is set at a diode voltage drop which is typically equal to athreshold voltage of that diode junction, V_(th), from the positivepower supply. If the drain terminal 310 of the PMOSFET (which is alsothe source terminal 322 of the NMOSFET) were to go above V_(DD)+V_(th)such that the parasite PN junction of the PMOSFET turns on, then thegate to source voltage V_(GS) of the NMOSFET becomes too low for theNMOSFET to turn on. In that case, the NMOSFET turns off; and the NMOSFETwhich is in the path of the parasitic PN junction of the PMOSFET blocksthe unwanted current flow through that parasitic PN junction. Thus,unwanted current, caused by the high positive voltage build-up on thecase of the implantable pulse generator 108, is prevented.

In this manner, the current limiting circuit 300 blocks any unwantedcurrent flow that may be induced in the conductive loop formed by theimplanted medical device. Thus, when this circuit is inserted in thatconductive loop, the switch 302 is kept off to introduce and maintain ahigh impedance within that conductive loop.

Thus far, the over-sense control signal on the second control terminal338 has been tied low. In that state, only the first and second controlsignals dictate whether the switch 302 is on or off. Those controlsignals close the conductive loop when stimulation signals are appliedon the electrically excitable tissue for a therapeutic effect and openthe conductive loop otherwise. However, with those signals alone, theconductive loop is closed to stimulate the electrically excitable tissuefor therapeutic effect even when a significant level of external signalis present When an external signal is applied on that closed conductiveloop, unwanted current may be induced in the conductive loop which cancause undesirable physiological effects.

Thus, the conductive loop is opened when an external signal is presentno matter what the first and second control signals may be. Theover-sense control signal applied on the second control terminal 338allows for an override of the first and second control signals. Thusfar, the over-sense control signal has been tied low such that only thefirst and second control signals dictate whether the switch 302 is on oroff.

However, when the over-sense control signal is set high, this signalsets the output of the third NOR gate 344 high no matter what the firstand second control signals may be. As discussed previously, when theoutput of the third NOR gate is high, the switch turns off andintroduces a high impedance into the conductive loop to open theconductive loop. Thus, by setting the over-sense control signal appliedon the second control terminal 338 high, the conductive loop is openedno matter what the first and second control signals may be. In thismanner, the conductive loop is opened when a significant level of anexternal signal such as a time-alternating electromagnetic field ispresent even if the conductive loop were closed for therapeutic effect.

This over-sense control signal can be generated by two means. First, theuser can actively set the over-sense control signal high when the useris about to enter an area where the level of external signal is likelyto be greater than an external signal threshold level. For example, inthe implanted medical device system 100 of FIG. 1B, the user can send acontrol signal via the programmer 112 to set the over-sense controlsignal high. In this manner, the user controls the opening of theconductive loop before the external signal having a level which isgreater than the external signal threshold level causes unwantedphysiological effects.

Alternatively, a high over-sense control signal is automaticallygenerated when an external signal is sensed by using an external signalsensor. Such a sensor can include a field sensing coil, a chatteringreed switch, or a hall effect sensing switch. These sensors are commonlyavailable and act as a switch to toggle between two states when thelevel of a magnetic field is greater than the external signal thresholdlevel and when the level of a magnetic field is less than the externalsignal threshold level.

In an alternative embodiment of the present invention, the externalsignal sensor is implemented using CMOS technology devices. Such anexternal signal sensor circuit 400 is shown in FIG. 4. This circuitincludes a first NMOSFET device 402 and a second NMOSFET device 404 thatform matched constant current sources. The gate terminals of both ofthose devices are tied to a biasing circuit that can provide a NBIASsignal to appropriately bias those NMOSFET devices according to thecommon art of transistor circuit design. A first PMOSFET device 406 anda second PMOSFET device 408 form a current source mirror pair that alsooperates as a common gate differential current amplifier according tothe common art of transistor circuit design. The source terminal 410 ofthe second PMOSFET 408 is operatively connected to the node 330 of FIG.3.

A NAND-gate 412 and a first inverter 414 effectively form an AND gatewith the output of that gate being operatively connected to the commonsource node of NMOSFETs 402 and 404. A first NAND input terminal 416 istied to the first control terminal 336 of FIG. 3, and a second NANDinput terminal 418 is tied to the third control terminal 342 of FIG. 3.

The output of the differential amplifier pair of PMOSFETs is at thedrain terminal of the second PMOSFET 410. A positive feedback loop,including a second inverter 420, a third inverter 422, and a chargingcapacitor 424, is operatively connected between the output of thedifferential amplifier and a over-sense node 426. This over-sense nodeis operatively connected to the second control terminal 338 of thecurrent limiting circuit 300 of FIG. 3 and provides the over-sensecontrol signal. A current pulling NMOSFET 428 is operatively connectedat the output of the differential amplifier. The gate of that device isalso tied to the biasing circuit that can provide the NBIAS signal toappropriately bias that NMOSFET.

The operation of the external signal sensor circuit 400 is now describedreferring to both FIGS. 3 and 4. The first NAND input terminal 416 whichis tied to the first control terminal 336 of FIG. 3 receives the firstcontrol signal. The second NAND input terminal 418 which is tied to thethird control terminal 342 of FIG. 3 receives the second control signal.The overall functional purpose of the external signal sensor circuit isto automatically open the conductive loop formed by an implanted medicaldevice when the level of an external signal is greater than the externalsignal threshold level. Thus, the operation of this circuit is notcritical when the conductive loop is already open in the case theconductive loop is not stimulating the electrically excitable tissue fortherapeutic effect.

In that case, the first and second control signals are both set high,and the output of the first inverter 414 is set high. Because of a highvoltage at the source terminals of the NMOSFETs 402 add 404 of thematched constant current sources, those NMOSFETs are turned off whichconsequently turns off the pair of PMOSFETs 406 and 408. The currentpulling NMOSFET 428 is still on and discharges the output node of thedifferential amplifier which in turn sets the over-sense node 426 low.The over-sense signal is set low because with the first and secondcontrol signals set high, the conductive loop is already opened.

In contrast, in the case the conductive loop is already closed tostimulate the electrically excitable tissue for therapeutic effect, theoutput of the first inverter 414 is set low since either the firstcontrol signal or the second control signal is set low. With a lowvoltage at the source terminals of the NMOSFETs 402 and 404, thoseNMOSFETs are turned on. Consequently, the pair of PMOSFETs 406 and 408are turned on. Thus, the matched constant current sources of theNMOSFETs 402 and 404 are appropriately biased to conduct current.

Note that the source terminal of the second PMOSFET 408 is operativelyconnected to the node 330 of FIG. 3. This node has a change in voltageif an external signal is present to cause a change in the currentflowing in the conductive loop. Thus, the external signal sensor circuit400 detects a significant level of unwanted current in the conductiveloop that indirectly indicates the presence of a significant level ofexternal signal.

Typically, the external signal is an AC signal, and the voltage changeat the node 330 goes up and down along with the external AC signal. Inthe external signal sensor circuit 400 of FIG. 4, the differentialamplifier senses a first positive change in the voltage at the node 330.With that positive voltage change, the current flowing through thesecond PMOSFET 408 increases. This increase in current charges up theoutput node of the differential amplifier pair to drive the over-sensenode 426 high When the level of external signal is above an externalsignal threshold level, the voltage on the over-sense node charges up toa high enough level which opens the conductive loop when applied on thesecond control terminal 338 of FIG. 3.

The positive feedback loop, which includes the second inverter 420, thethird inverter 422, and the charging capacitor 424, provides positivefeedback for coupling any positive change at the output of thedifferential amplifier to the over-sense node 426. This positivefeedback loop, which may include a Schmitt trigger inverter as one ofthe second and third inverters, can cause an immediate voltage change onthe over-sense node when the voltage goes positive. The chargingcapacitor 424 in this loop keeps the over-sense node positive once thatnode has gone positive even when the AC external signal causes anegative voltage change at the node 330 of FIG. 3.

In this manner, the external signal sensor circuit 400 of FIG. 4 cangenerate an over-sense control signal to be applied on the secondcontrol terminal 338 of the current limiting circuit 300 of FIG. 3. Theexternal signal sensor circuit 400 of FIG. 4 determines when asignificant level of external signal is present to automatically ensurethat the conductive loop is opened. Thus, unwanted current that may beinduced in the conductive loop by the external signal is automaticallylimited using the external signal sensor circuit.

The advantages of the invention described herein can be generalized toany partially or fully implanted medical device that forms a conductiveloop with the electrically excitable tissue. In addition, the inventioncan be generalized to medical devices implanted in any living organismhaving electrically excitable tissue that can be electrically stimulatedfor therapeutic effect. Moreover, the invention can be generalized toany means for introducing a high impedance into that conductive loopwhen an external signal such as a time-alternating electromagnetic fieldis present. Also, the invention can be generalized to prevent a build-upof any level of unwanted voltages in the implanted medical device. Thelevel of unwanted voltages that can be handled by the present inventionis limited by the breakdown voltage of the specific technology that isused in the implementation of the present invention. Accordingly, theforgoing description is by way of example only. The invention is limitedonly as defined in the following claims and equivalents thereof.

I claim:
 1. A method for limiting unwanted current, induced when thelevel of an external signal is greater than an external signal thresholdlevel, in a conductive loop formed by a medical device implanted withina living organism having electrically excitable tissue, the methodincluding the steps of: A. closing said conductive loop when saidmedical device is stimulating said electrically excitable tissue forachieving a therapeutic effect if the level of said external signal isless than said external signal threshold level; and B. opening saidconductive loop by generating and maintaining a high impedance withinsaid conductive loop whenever the level of said external signal isgreater than said external signal threshold level.
 2. The method forlimiting unwanted current as recited in claim 1, further including thestep of: C. sensing said external signal to automatically perform stepB.
 3. The method for limiting unwanted current as recited in claim 2,wherein said step C includes the step of: D. sensing the level of saidunwanted current induced in said conductive loop, wherein a significantlevel of said unwanted current in said conductive loop indicates thatthe level of said external signal is greater than said external signalthreshold level.
 4. The method for limiting unwanted current as recitedin claim 1, wherein said medical device includes stimulation electrodes,implanted near said electrically excitable tissue, and an implantablepulse generator operatively connected to said stimulation electrodes viaa lead wire, said implantable pulse generator having a case that acts asa reference electrode with respect to said stimulation electrodes.
 5. Acurrent limiting apparatus that is adapted to be operatively connectedas part of a conductive loop formed by a medical device implanted withina living organism having electrically excitable tissue, said currentlimiting apparatus comprising: a switch, adapted to be operativelyconnected within said conductive loop, that turns on to close saidconductive loop and that turns off to open said conductive loop; andmeans for controlling said switch to turn said switch on when saidmedical device is stimulating said electrically excitable tissue toachieve a therapeutic effect if the level of an external signal is lessthan an external signal threshold level, and to turn said switch offwhenever the level of said external signal is greater than said externalsignal threshold level, said external signal being an induced currentcaused by said conductive loop.
 6. The current limiting apparatus ofclaim 5, wherein said switch introduces and maintains a high impedancewithin said conductive loop when the level of said external signal isgreater than said external signal threshold level.
 7. The currentlimiting apparatus of claim 5, further comprising: an external signalsensor for sensing said external signal, wherein said means forcontrolling turns said switch off when said sensor senses that the levelof said external signal is greater than said external signal thresholdlevel.
 8. The current limiting apparatus of claim 7, wherein saidexternal signal sensor senses the level of said unwanted current inducedin said conductive loop, wherein a significant level of said unwantedcurrent in said conductive loop indicates that the level of saidexternal signal is greater than said external signal threshold level. 9.The current limiting apparatus of claim 5, wherein said medical deviceincludes stimulation electrodes, implanted near said electricallyexcitable tissue, and an implantable pulse generator operativelyconnected to said stimulation electrodes via a lead wire, saidimplantable pulse generator having a case that acts as a referenceelectrode with respect to said stimulation electrodes.
 10. A currentlimiting apparatus that is adapted to be operatively connected as partof a conductive loop formed by a medical device implanted within aliving organism having electrically excitable tissue, said currentlimiting apparatus comprising: means for switching said conductive loopbetween being closed and being open; and a control circuit, operativelyconnected to said means for switching, that controls said means forswitching to close said conductive loop when said medical device isstimulating said electrically excitable tissue to achieve a therapeuticeffect if the level of an external signal is less than an externalsignal threshold level, and that controls said means for switching toopen said conductive loop whenever the level of said external signal isgreater than said external signal threshold level, said external signalbeing an induced current caused by said conductive loop.
 11. The currentlimiting apparatus of claim 10, wherein said means for switchingintroduces and maintains a high impedance within said conductive loopwhen the level of said external signal is greater than said externalsignal threshold level.
 12. The current limiting apparatus of claim 10,further comprising: means for sensing said external signal, wherein saidcontrol circuit controls said means for switching to open saidconductive loop when said means for sensing senses that the level ofsaid external signal is greater than said external signal thresholdlevel.
 13. The current limiting apparatus of claim 10, wherein saidmedical device includes stimulation electrodes, implanted near saidelectrically excitable tissue, and an implantable pulse generatoroperatively connected to said stimulation electrodes via a lead wire,said implantable pulse generator having a case that acts as a referenceelectrode with respect to said stimulation electrodes.
 14. A method forlimiting unwanted current, induced when the level of an external signalis greater than an external signal threshold level, in a conductive loopformed by a medical device implanted within a living organism havingelectrically excitable tissue, the method including the steps of: A.closing said conductive loop when said medical device is stimulatingsaid electrically excitable tissue for achieving a therapeutic effect ifthe level of said external signal is less than said external signalthreshold level; B. opening said conductive loop by generating andmaintaining a high impedance within said conductive loop whenever thelevel of said external signal is greater than said external signalthreshold level and generates a voltage build-up to a positive polarityat a node within said conductive loop; and C. opening said conductiveloop by generating and maintaining a high impedance within saidconductive loop whenever the level of said external signal is greaterthan said external signal threshold level and generates a voltagebuild-up to a negative polarity at said node within said conductiveloop.
 15. A current limiting apparatus that is adapted to be operativelyconnected as part of a conductive loop formed by a medical deviceimplanted within a living organism having electrically excitable tissue,said current limiting apparatus comprising: a sensor for detecting avoltage build up in said conductive loop to either one of a positivepolarity and a negative polarity when the level of an external signal isgreater than a threshold level; a switch, adapted to be operativelyconnected within said conductive loop, that turns on to close saidconductive loop and that turns off to open said conductive loop; and acontrol circuit, operatively coupled to the sensor and the switch, forcontrolling said switch to turn said switch on when said medical deviceis stimulating said electrically excitable tissue to achieve atherapeutic effect if the level of said external signal is less than anexternal signal threshold level, and to turn said switch off wheneverthe sensor detects the voltage build up in the conductive loop to eitherone of the positive polarity and the negative polarity when the level ofsaid external signal is greater than said external signal thresholdlevel.
 16. A method for limiting unwanted current, induced when thelevel of an external signal is greater than an external signal thresholdlevel, in a conductive loop formed by a medical device implanted withina living organism having electrically excitable tissue, the methodincluding the steps of: A. closing said conductive loop when saidmedical device is stimulating said electrically excitable tissue forachieving a therapeutic effect if the level of said external signal isless than said external signal threshold level; and B. opening saidconductive loop by generating and maintaining a high impedance withinsaid conductive loop whenever the level of said external signal isgreater than said external signal threshold level, said step B fartherincluding the steps of: opening a first switch device coupled withinsaid conductive loop whenever the level of said external signal isgreater than said external signal threshold level; and opening a secondswitch device coupled within said conductive loop whenever the level ofsaid external signal is greater than said external signal thresholdlevel, wherein, the first switch device and the second switch device areoperatively coupled in series within the conductive loop such that thefirst switch device prevents parasitic current flow through said secondswitch device when the external signal causes voltage build-up to apositive polarity within the conductive loop, and such that said secondswitch device prevents parasitic current flow through said first switchdevice when the external signal causes voltage build-up to a negativepolarity within the conductive loop.